Field of the Invention
The present invention relates to a technique for developing an HDR (high dynamic range) image.
Description of the Related Art
Due to, for example, progress in LED elements for displays in recent years, it is possible to have blacker blacks and higher luminances, and it is possible to display data for a high dynamic range (hereinafter referred to as HDR) image unchanged without compressing it. With HDR, it is possible to realistically reproduce scenery such as the neon of a night scene or the tones of clouds in a blue sky for which contrast is poor with a conventional dynamic range (hereinafter referred to as SDR (Standard Dynamic Range)).
However, HDR also has a characteristic where bright portions are glaringly bright, and a main object is perceived as being relatively dark. This is thought to be due to simultaneous contrast which occurs in cases of spatial adjacency, in color contrast which is a characteristic of visual perception for a human. In simultaneous contrast, there is brightness contrast as well as chroma contrast where the chroma of a color that is surrounded by a color having a high chroma appears reduced.
The aforementioned phenomenon of simultaneous contrast is one where a brightness/darkness difference is enhanced, and, while there are scenes where this is desirable for realistic HDR image quality, there are cases where a desirable image is not necessarily achieved if the brightness of a main object, in particularly a person's face or the like, has darkened. In such a case, it is envisioned that a user will perform an adjustment to lower the contrast by using an image quality adjustment function inside a camera at a time of capturing, or an image quality adjustment function inside a PC application at a time of development.
As a typical method for contrast adjustment, there is a common method for performing amplitude control that takes a signal level as a reference, and adjusts signals that are larger than this and signals that are smaller than this. However, with a human's vision characteristics, because a sensory amount is perceived as proportional to the logarithm of the intensity of a stimulus amount (a luminance value in this case), even if adjustment amounts for a dark portion and a bright portion with respect to a reference signal level are made to be the same amounts, the same amount of change will not be perceived for the dark portion and the bright portion. This phenomenon becomes more noticeable when a target luminance as handled with HDR is a high luminance that exceeds 1000 nits.
Accordingly, for example, Japanese Patent Laid-Open No. 2016-086347 proposes a correction method that maintains a contrast by compressing based on global luminance when the dynamic range of an output apparatus is smaller than the dynamic range of an image capturing apparatus. In addition, Japanese Patent Laid-Open No. 2001-027890 proposes a method for performing luminance adjustment of a backlight so that an average luminance level does not change visually, while keeping a correlation with a contrast adjustment performed by contrast adjustment means.
However, the foregoing conventional technique disclosed by Japanese Patent Laid-Open No. 2016-086347 is a method for maintaining contrast as much as possible while considering the characteristic of visual perception of a human, in a method of compressing a dynamic range from a wide state to a narrow state. However, this is not a method for guaranteeing that adjustment amounts for dark portions and bright portions will be perceived as equivalent amounts.
In addition, the conventional technique disclosed by Japanese Patent Laid-Open No. 2001-027890 is characterized by attempting to maintain an average luminance with consideration given to a luminance reproduction range of a backlight of a display apparatus, in addition to simple amplitude control of a signal. However, this is not a method for guaranteeing that adjustment amounts for dark portions and bright portions will be perceived as equivalent amounts.